The corrosion of metals and reinforced concrete has been a persistent problem in our modern post industrial revolution world that has cost billions of dollars and many lives. For instance in 1967 the sudden collapse because of corrosion fatigue of the Silver Bridge over the Ohio River at Point Pleasant, Ohio resulted in the loss of 46 lives and cost millions of dollars. In 1983, the Michael L. Morano Bridge crossing the Mianus River on I-95 in Connecticut collapsed because of corrosion. Three people were killed in that collapse and the tragedy spurred a reaction by the Connecticut General Assembly to establish a $5.5 Billion bridge and highway renovation program. The collapse of the Algo Centre Mall's roof in Elliot Lake Ontario in June of 2012, was found to have been caused by the corrosion of a steel beam due to water leakage that brought a portion of the concrete structure crashing down two stories. That collapse killed two women and injured 20 more.
The corrosion process of reinforcing steel that takes place in concrete is electrochemical. When electrons commence flowing between anodic and cathodic sites on the reinforcing rod in the concrete the process of corrosion begins. Corrosion requires four basic elements in order to occur: 1) an anode site where current flows from and corrosion takes place, i.e. a site on the steel reinforcing rod; 2) a cathode site where no corrosion occurs and to which current flows; 3) an electrolyte medium capable of conducting electric current by ionic current flow such as soil, water or concrete; and 4) a metallic pathway or connection between the anode and cathode, which permits the electrical current to return and complete the circuit. By design the reinforcing steel in concrete should not normally corrode because of the formation of a passive oxide film on the surface of the steel. The hydration of cement in freshly poured concrete has a very high alkalinity that reacts with oxygen to stabilize the passive oxide film on the surface of concrete embedded reinforcing rod. The passive oxide film should provide continued protection so long as the alkalinity is maintained. Concrete typically has a pH above 12 largely due to the presence of sodium hydroxide, calcium hydroxide and potassium hydroxide. Despite these facts, how this passive oxide film protects the metal is not known, it appears to isolate the metal from the environment and thereby retards the corrosion of the metal as long as the film is intact.
The two most significant causes of the corrosion of reinforcing steel are chloride contamination and carbonation. Chloride contamination caused corrosion of reinforcement steel is well documented. In most instances chloride ions enter into the concrete as a result of de-icing salts or from seawater in marine environments. The other common sources of chloride contamination are: 1) contaminated aggregates and/or mixing water, 2) chloride containing admixtures which are used to accelerate curing of concrete; 3) air born salts; 4) salts in ground water; and 5) the salts in chemicals that may be applied to concrete surfaces. When chloride ions are present in sufficient quantity they will chemically disrupt the passive oxide film on the reinforcing steel and corrosion will then take place.
Carbonation of a reinforced concrete structure takes place when CO2 from the atmosphere diffuses through the porous concrete. Once diffused in the concrete the CO2 neutralizes the alkalinity of the concrete. The pH of 12 necessary to maintain the passive oxide film on the reinforcing steel will drop dramatically as a result of carbonation to as low as 8 resulting in a destabilization of the passive oxide film. Without the protection of the passive oxide film oxygen and water present in the concrete will cause the metal to corrode. Carbonation is typically a slow process that depends primarily on the porosity and permeability of the concrete. It is rarely a problem on structures that are built with good quality concrete with adequate depth of cover over the reinforcing steel.
The prior art teaches many methods and devices engineered to slow down the corrosion process of metals in reinforced concrete structures. Cathodic protection (hereinafter “CP”), however, is the only method proven to stop corrosion in existing reinforced concrete structures. In the art it is understood that CP is defined as the reduction or elimination of corrosion by making the metal a cathode via an impressed direct current (DC), or by connecting it to a sacrificial or galvanic anode. CP is achieved in two basic ways. The metal being protected is made into a cathode by either: 1) impressing a direct current (DC); or 2) by connecting the metal to a sacrificial or galvanic anode such as disclosed in U.S. Pat. No. 5,341,562, Furuya, et al. and U.S. Pat. No. 7,160,433, Bennett. The cathodic areas of the metal become an electrochemical cell that will not corrode. The prior art teaches that if all the anode sites were forced to function as current-receiving cathodes then the entire metallic structure would become a cathode and corrosion would be prevented.
CP has been used successfully to protect bridges, underground pipelines, ship hulls, offshore oil platforms, underground storage tanks, and countless other structures that are often exposed to corrosive environments. The effective use of CP on a concrete bridge structure has been known since 1973. As noted above the corrosion process itself generates an electric current. To counteract the current generated by the corrosion CP supplies a source of external current thereby eliminating the corrosion.
It is well known in the art that the life expectancy of an impressed current CP system is much greater than sacrificial or galvanic anodes. For instance a conductive coating sacrificial or galvanic anode system in a marine environment is only expected to last less than 10 years as opposed to a titanium mesh impressed current CP which can function effectively for more than 75 years. Sacrificial or galvanic anodes in turn have the advantage of not requiring an impressed current or battery power supply once the system is charged. Sacrificial or galvanic anodes also utilize less expensive metals such as zinc which in some systems can be replaced once the sacrificial or galvanic anode is decayed beyond its effectiveness. The current jacketed systems that are in use in the art do not provide a reliable method of maintaining either the impressed current systems or the sacrificial or galvanic anodes in place during installation to insure proper function to prevent corrosion of the structure sought to be cathodically protected. The common problem in the current art of deformation of the titanium mesh in the impressed current systems and the zinc mesh of the sacrificial or galvanic anodes systems during the jacket infusion of concrete in particular reduces and in some cases eliminates the effectiveness of the system.
My invention utilizes a jacketed CP system that is capable of effectively utilizing either an impressed current system or a sacrificial or galvanic anode or both in combination. An impressed current CP system for concrete structures is typically comprised of the following basic components: 1) a DC power supply known as a rectifier; 2) an inert anode material such as catalyzed titanium anode mesh; 3) wiring and conduit; and 4) some form of instrumentation such as an embedded silver/silver-chloride reference electrode. A CP sacrificial or galvanic anode system for reinforced concrete uses a more reactive metal (anode) such as zinc or aluminum-zinc-indium (Al—Zn—In), to create a current flow. Sacrificial anode systems are based on the principle of dissimilar metal corrosion and the relative position of different metals in the galvanic series. The direct current is generated by the potential difference between the anode and reinforcing steel when connected and requires no external power supply. The sacrificial anode will corrode during the process and is consumed. Current will flow from the anode, through the concrete, to the corroding reinforcing steel.
While the electrochemistry of impressed current and sacrificial or galvanic anode CP systems are well known in the prior art the challenge in constructing an effective CP system lies in the manner in which such devices attach to a structure while maintaining effective electrical connections that can reliably maintain the metal or reinforced concrete structure being protected as a cathode. Several impressed current CP systems exist in the prior art. For example in U.S. Pat. No. 4,080,272, Ferry, et al. teaches a method and apparatus for determining the true cathode polarization potential for automatically regulating the impressed current in a cathodic protection system; and U.S. Pat. No. 8,557,089, Schutt teaches a cathodic protection system for marine applications utilizing an impressed current. Similarly, several sacrificial or galvanic anode CP systems also exist in the prior art. For example in U.S. Pat. No. 7,704,372, Glass et al. teaches a sacrificial anode assembly in the form of a cell. Additionally, the combination of impressed current and sacrificial or galvanic anodes in one CP system is also taught in U.S. Pat. No. 8,999,137, Glass et al.
Typically when an existing structure is sought to be protected by an impressed current CP system a fiberglass reinforced plastic form (FRP form) is placed around the structure that is being protected by the device. The FRP form has disposed within it a titanium mesh connected to a current distribution titanium bar that will be connected to the DC power source or rectifier. Once the FRP form is secured in place it is filled with concrete that is typically pumped into the form. In like manner, typically when an existing structure is sought to be protected by a sacrificial or galvanic anode CP system a fiberglass reinforced plastic form (FRP form) is also placed around the structure that is being protected by the device. The FRP form has disposed within it a zinc or aluminum-zinc-indium mesh that will act as the sacrificial anode in the system. In this system as well once the FRP form is secured in place it is filled with concrete that is typically pumped into the form. A CP system that utilizes both an impressed current and a sacrificial or galvanic anode would have both the titanium and the zinc or aluminum-zinc-indium mesh disposed within the FRP form. Several problems have plagued current art jacketed CP systems such as achieving an even flow of the concrete inside the FRP form with the mesh attached to it without damaging the mesh. Another common problem in the prior art is that the mesh will often create a short circuit by touching the inside reinforcement steel because there is no effective means for retaining the mesh in optimal placement within the jacket during the installation process.
Prior art systems do not maintain the mesh at the proper distance from the steel reinforcement that is being protected. Another common problem with the prior art occurs during assembly and pumping of the concrete into the FRP form when the mesh folds over at the corners and joints of the FRP form toward or away from the steel reinforcement sought to be protected. Prior art systems also lack an effective bottom form or locking system at the joints of the FRP form necessary to prevent concrete leaks as the form is being pumped full of concrete. Also FRP forms of prior art systems will often disengage and separate from the concrete due to the lack of an effective attachment means between the interior surface of the FRP form and the concrete.